Heretofore, the grinding of a lawn mower reel has been a tedious and time consuming project for a number of reasons. There has never been a device which was capable of both single blade grinding and spin grinding of the entire lawn mower reel, without removing it from its supporting assembly. In prior art devices, only single blade grinding of the mower reel could take place, either by removing the reel from its supporting assembly or by mounting the assembly complete with reel in the grinding machine.
Alternatively, another prior art machine provides for spin grinding of the reel but only if the reel is removed from its supporting assembly. All single blade grinding, whether the reel is removed from its frame for the purpose or not, produces a result which is rarely accurate enough to give satisfactory reel/cutter bar engagement. It is necessary for proper cutting to provide tolerances in the order of 0.001 of an inch and slight misalignment can cause poor cutting performance and increased wear on both the mower reel and cutting bar.
Prior art machines have provided for single blade grinding and spin grinding although no single machine has to date been perfected which could perform both operations, without removing the reel from its assembly. For example, U.S. Pat. No. 2,279,798 to Shelburne discloses a device capable of single blade grinding while U.S. Pat. No. 2,314,945 to Maynard, et al. discloses a device which can only spin grind a mower reel. As stated previously, the machines disclosed by Shelburne and Maynard require disassembly of the lawn mower structure for the grinding machines disclosed therein are capable of only accepting the reel portion of the lawn mower.
A device which can both single blade grind and spin grind a lawn mower reel without removing it from its frame provides for a degree of flexibility far superior to any sharpening technique available before the present invention. Each blade of a lawn mower reel must not only be spun ground but also cut to provide a relief angle on the surface of the blade which contacts the cutting bar. In prior art grinding operations, once this was accomplished, the reel was removed from the grinding machine and attached to the lawn mower assembly containing a cutting bar. Lapping paste would be placed upon the reel and the reel revolved backwards mechanically so as to obtain a proper cutting relationship between the individual cutting blades of the reel and the cutting bar. Alternatively, if the reel had been single blade ground while still mounted in its frame, the mower assembly was removed from the grinding machine, the cutting bar was attached, and the same lapping operation performed.
In spin grinding devices, such as those disclosed by Maynard et al., although the necessity for lapping is diminished by the spin grinding cycle, there was no way in which such a device could provide the necessary relief angle on the individual cutting blades. Some lapping may nevertheless be necessary even after spin grinding with such a machine for, as was stated previously, the cutting reel must be removed from the lawn mower assembly in order to fit within the spin grinding machine and reassembling the lawn mower structure would more times than not result in some misalignment which may require a lapping operation to get proper fit between the lawn mower reel and cutting bar.
In the device of the present invention, a relief angle can be cut within each cutting blade while a spin grinding operation can further be carried out in order to eliminate the need for lapping. There are also no reassembly tolerance problems for the entire lawn mower assembly is never disassembled. Alternatively, the spin grinding operation can be carried out firstly, followed by the single blade grinding operation to produce the relevant relief angle to each individual blade.